This section provides background information related to the present disclosure which is not necessarily prior art.
Articles of footwear conventionally include an upper and a sole structure. The upper may be formed from any suitable material(s) to receive, secure, and support a foot on the sole structure. The upper may cooperate with laces, straps, or other fasteners to adjust the fit of the upper around the foot. A bottom portion of the upper, proximate to a bottom surface of the foot, attaches to the sole structure.
Sole structures generally include a layered arrangement extending between a ground surface and the upper. One layer of the sole structure includes an outsole that provides abrasion-resistance and traction with the ground surface. The outsole may be formed from rubber or other materials that impart durability and wear-resistance, as well as enhancing traction with the ground surface. Another layer of the sole structure includes a midsole disposed between the outsole and the upper. The midsole provides cushioning for the foot and is generally at least partially formed from a polymer foam material that compresses resiliently under an applied load to cushion the foot by attenuating ground-reaction forces. The midsole may define a bottom surface on one side that opposes the outsole and a footbed on the opposite side that may be contoured to conform to a profile of the bottom surface of the foot. Sole structures may also include a comfort-enhancing insole and/or a sockliner located within a void proximate to the bottom portion of the upper.
Midsoles using polymer foam materials are generally configured as a single slab that compresses resiliently under applied loads, such as during walking or running movements. Generally, single-slab polymer foams are designed with an emphasis on balancing cushioning characteristics that relate to softness and responsiveness as the slab compresses under gradient loads. Polymer foams providing cushioning that is too soft will decrease the compressibility and the ability of the midsole to attenuate ground-reaction forces after repeated compressions. Conversely, polymer foams that are too hard and, thus, very responsive, sacrifice softness, thereby resulting in a loss in comfort. While different regions of a slab of polymer foam may vary in density, hardness, energy return, and material selection to balance the softness and responsiveness of the slab as a whole, creating a single slab of polymer foam that loads in a gradient manner from soft to responsive is difficult to achieve.
Corresponding reference numerals indicate corresponding parts throughout the drawings.